Distinct Features of the Summer Arctic Oscillation

The Summer Arctic Oscillation (SAO) has received little attention compared with the Winter Arctic Oscillation (WAO). However, recent studies have reported that the SAO is associated with extreme heat events in the Northern Hemisphere. Using reanalysis and satellite datasets, this study investigates the spatial pattern and dynamical mechanisms of the SAO. While the WAO represents the meridional displacement of the eddy-driven jet at;408N, the SAO shows zonal wind anomalies, one in the subtropics (;308N) and the other along the Arctic coast (;758N). The center of action in the subtropics is mainly driven by meridional wave propagation, while that near the Arctic is largely dominated by baroclinic eddy generation at the surface. The findings here suggest that Arctic cloud cover plays an important role in the SAO, which increases the low-level meridional temperature gradient through the shortwave cloud radiative effect during the positive SAO phase. Arctic sea ice anomalies show a positive correlation with the SAO across most of the Arctic Ocean, partly offsetting the cloud radiative effect. The SAO index shows a strong positive correlation with the cloud cover in the Arctic and upper-level zonal wind along the Arctic coast, showing a clear double jet structure in the Northern Hemisphere at the positive SAO phase. This study suggests a new perspective on annular mode dynamics in Northern Hemisphere summer and the need for more studies on the summer jet dynamics distinct from winter.